METHODS OF TESTING THE ACIDITY OF MILK 135 



has been completely neutralized, that is, changed into 

 sodium lactate. 



Having completed the neutralization of the acid, 

 we examine the burette or the graduated cylinder con- 

 taining the alkali to find exactly how much alkali so- 

 lution has been used in neutralizing the acid. The 

 lactic acid has required, say, ,28 cc. of soda solution. 

 Each cubic centimeter of alkali neutralized by acid 

 corresponds to .009 gram of acid, and 28 cc. would 

 therefore correspond to .25 gram of lactic acid. This 

 amount of lactic acid in 100 cc. is .25 per cent. The 

 process of making a chemical determination by means 

 of a standard solution is known as titration. 



NEUTRALIZATION METHOD APPLIED TO TEST- 

 ING ACIDITY OF MILK AND MILK PRODUCTS 



In practical dairy work, one is freed from the ne- 

 cessity of preparing standard solutions, except in a 

 simple way, and the calculations needed to figure the 

 results are direct and easy. The caustic soda solution 

 is prepared in such strength that i cc. of it equals .1 or 

 o.oi per cent, of lactic acid, when a certain amount of 

 milk or other substance is used. All tests for the acid- 

 ity of milk and its products are based upon the gene- 

 ral principles previously described and differ from one 

 another simply in some of the details of carrying out 

 the process. There are now available several forms 

 of so-called acid tests. 



The essential apparatus and reagents for determining 

 acidity in milk and its products are the following : ( I ) 

 An accurately graduated burette or cylinder for meas- 

 uring the amount of standardized alkali solution used ; 



